CDMA cellular systems are currently in widespread use throughout North America providing telecommunications to mobile users. In order to meet the demand for transmission capacity within an available frequency band allocation, CDMA cellular systems divide a geographic area to be covered into a plurality of cell areas. Within each cell is positioned a base station with which a plurality of mobile stations within the cell communicate.
In general, it is desired to have as few base stations as possible. This is because base stations are expensive and require extensive effort in obtaining planning permission. In some areas, suitable base station sites may not be available. In order to have as few base stations as possible, each base station ideally has as large a capacity as possible in order to service as large a number of mobile stations as possible. Several key parameters that determine the capacity of a CDMA digital cellular system are: processing gain, ratio of energy per bit to noise power, voice activity factor, frequency reuse efficiency and the number of sectors in the cell-site antenna system.
AABS (Adaptive Antenna Beam Selection) is a method used in CDMA cellular Base Stations to improve traffic capacity in “hot spot” sectors without requiring additional carriers (i.e. more spectrum) at the hot spot. This spectrally efficient technique replaces the standard sector antenna beam pattern with a multiplicity, typically three, of beams per sector. These new beams have higher directivity on both the forward and reverse links. This higher directivity reduces the forward interference seen by a mobile terminal and reduces the received interference level at the base station's receiver. Consequently, the RF power required to support a typical call in the forward ink is lower than that required for a conventional antenna beam. This results in a significantly greater number of AABS calls being supportable with a base station's limited transmitter power than is possible with a conventional sector beam.
In a similar manner to the forward link situation, the reverse link AABS beams are more directive than a conventional sector beam. As a result, the mobile terminal's RF power required to support a typical call in an AABS sector will be lower than for a conventional sector call. This will also help prolong the mobile terminal's battery life.
An example of the AABS method of achieving an increase in capacity is shown in FIG. 1 in which a single wide beam width antenna per sector is replaced with an antenna array that allows the formation of a number of narrower beam widths that cover the area of the original beam. Referring to FIG. 1, a conventional CDMA communication cell 100 is shown comprising three adjacent sectors, alpha 102, beta 104 and gamma 106. Each cell comprises an antenna tower platform 120 located at the intersection of the three sectors. The antenna tower platform 120 has three sides forming an equilateral triangle. Each sector has three antennas. Only the antennas in sector alpha 102 are shown, and these consist of a first antenna 114, a second antenna 116 and third antenna 112 mounted on a side of the antenna tower platform 120. Each sector also has three beams. Only the beams in sector alpha 102 are shown, and these consist of a first beam 108, a second beam 110 and a third beam 112. The three beams 108,110,112 are adjacent with some overlap. The three sectors alpha 102, beta 104 and gamma 106 are identical in structure with respect to antennas and beams. The signal for a particular user can then be sent and received only over the beam or beams that are useful for that user. If the pilot channel on each beam is unique (i.e. has a different PN (pseudo-random noise) offset) within each sector then the increase in capacity is limited due to interference between reused pilot channels in different cells.
An improvement is to use multiple narrow beams for the traffic channels and transmit the overhead channels (pilot, sync, and paging channels) over the whole sector so that the pilot channel is common to all the narrow beams used by the traffic channels in that sector. This leads to substantial gains in capacity. For example, a change from a system with a single beam per sector to a system with three beams per sector with a common pilot channel yields a 200 to 300% increase in capacity. It is therefore desirable that the pilot channel be broadcast over the area covered by the original wide beam. A possible arrangement is to use multiple beams per sector for the traffic channels and transmit the overhead channels over a separate wide beam antenna covering the whole sector. However, this requires the expense of extra hardware as well as the calibration and adjustment needed to match the phase of the pilot channel with the phase of the traffic channels over time and temperature.
Another possible solution is to use adaptive antenna array techniques to transmit and receive multiple narrow beams for the traffic channels and to transmit the overhead channels over the whole sector on the same antenna array. However, this requires complex calibration equipment and algorithms.
Yet another solution is to use an antenna array that transmits and receives multiple sectors over fixed narrow beams for the traffic channels and transmits the pilot channel on the same fixed narrow beams. However, the problem with this approach is that the strength of the pilot channel signal at any point in the sector is determined by the vector sum of all of the pilot channel signals from each beam. Since the pilot channel signals from each beam are coherent, areas where the vector sum of the pilot channel signals is null or severely degraded will occur. This can result in dropped calls when a mobile station enters one of these areas.
There is thus an advantage to provide an antenna array that uses fixed narrow beams for transmitting and receiving the traffic channels on multiple beams and can broadcast the common pilot channel over all of the sector using the same antenna array. Furthermore, it would be advantageous to provide an antenna system that did not require complex calibration and adjustment to maintain performance over time and temperature.
In CDMA cellular systems the (typically three) sectors at each site are identified by their PN offsets of the short code. The short code is a full length pseudo random sequence of 2^15-1 bits which repeats exactly 75 times every two seconds. All PN offsets are differentiated from each other by multiples of 64 chips which results in 512 PN offsets in total. In a similar manner to frequency re-use planning in a frequency division network, a CDMA network needs to have a PN plan which avoids PN ambiguities between different sectors and an effect known as PN pollution which can seriously impact a network's traffic capacity and dropped call rate.
Bearing in mind the limited number of PN offsets available to a network, one of the major advantages of AABS is that the increased number of beams utilized by the AABS sectors does not increase the number of PN offsets required by the network. Consequently there is no requirement for expensive and time consuming “optimization” when AABS sectors are introduced into a CDMA network.